Girder Tensile Stress in Precompressed Tensile Zone At Service III?

[BridgeNerdGuy]

The WSDOT Bridge Design manual [5.2.1.C] does not allow tension in bridge girders at the service III limit state, i.e. final stresses after losses with all permanent loads plus live load. AASHTO LRFD 9th edition Table [T5.9.2.3.2b-1] allows some tension in the precompressed tensile zone assuming uncracked sections.

I am typically designing with the 0.19λ(f'c)[sup]0.5[/sup]=0.19(1)(8.5)[sup]0.5[/sup] = 554PSI < 600 PSI limit for not worse than moderate corrosion conditions. The majority of the designs I have been involved in both classical prestressed girders and post-tensioned structures result in some tension during Service III with live loads.

WSDOT BDM says "The tensile stress in the precompressed tensile zone for the final service load condition (Service III) is limited to zero. This prevents cracking of the concrete during the service life of the structure and provides additional stress and strength capacity for overloads."

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[li]What are your thoughts? [/li]
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[ul]
[li]Does your DOT have a provision similar to WSDOT or do you hold you design to zero tension at the bottom of your section during Service III? (with live load)[/li]
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[ul]
[li]Do you think the zero limit is necessary to control cracking since this is more restrictive than AASHTO? Or is it potentially overly conservative?[/li]
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[Lomarandil]

It seems to me that WSDOT is acknowledging that their restriction is more conservative, and that's the way they want it.

 

[BridgeNerdGuy]

I must have found this topic more interesting than the other bridge engineers around here.

What intrigues me the most is that limiting designers to ZERO tensile stress at SERVICE III would often result in deeper girders, additional girders lines, and an increased prestressing steel quantity within girders. All of these things increase project cost and ultimately cost tax payers more dollars. Engineers are stewards of tax payer dollars and I'm not convinced this practice ultimately benefits the structure's performance or service life. It'll be interesting to see over time if more states adopt this practice, if AASHTO adopts more stringent criteria, or if we can tell structures designed in this manner actually benefit in a meaningful way.

 

[SWComposites]

Perhaps WA doesn't want its bridges falling down in earthquakes, hence the conservatism.